This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
WO 2008/055900 teaches a system for generation of encryption and decryption keys from an initial ‘master’ key. The system may advantageously be used in MPEG-4 to provide a receiver with a single key from which the decryption keys for lower enhancement layers may be generated. FIG. 1 illustrates the key generation for two times two (i.e. four) enhancement layers. A receiver that has access to key 2,2 may generate keys 1,2; 2,1; and 1,1 by applying one-way functions, advantageously hashes, to a specific part of the ‘master’ key. For example, to generate key 2,1, the receiver concatenates the left-hand side of key 2,2—L2,2—with the hash value—H(R2,2)—of the right-hand side of key 2,2—R2,2.
Similarly, to generate key 1,2, the receiver concatenates the right-hand side of key 2,2—R2,2—with the hash value—H(L2,2)—of the left-hand side of key 2,2—L2,2. Key 1,1 may be generated from either of keys 2,2; 2,1; and 1,2. The skilled person will appreciate that                Key 1,1=L1,2||H(R1,2)=H(L2,2)||H(R2,2) calculated via key 1,2, is equal to:        Key 1,1=H(L2,1)||R2,1=H(L2,2)||H(R2,2) calculated via key 2,1.        
The skilled person will also appreciate that as the functions that are used to modify the keys are one-way, it is not computationally feasible to generate key 3,2 from key 2,2 or from key 3,1 (although the opposite is possible).
As will be seen, WO 2008/055900 presents a drawback. It will therefore be appreciated that there is a need for a content protection system that overcomes the drawback of the prior art. This invention provides such a solution.